1. Field of the Invention
This invention relates to range processing systems and methods and more particularly to systems and methods for determining the range of a signal retransmitting target device such as a communications satellite.
2. Description of the Related Art
Control of communication satellites and other target devices which are capable of receiving and retransmitting ranging signals requires that the location of the target be known. The distance or slant range between the ranging station antenna and the target is one factor in determining the location of the target. Conventional ranging systems used for determining this distance can be characterized as either pulse or continuous wave type systems.
Pulse type systems determine the range of the target device directly by measuring the time required for a short pulse signal to be transmitted to the target and retransmitted back to the transmitting station. In a typical pulse system, a reference clock counter is started at the time of transmission of a pulse signal to the target device. When the pulse signal is returned to the transmitting station the counter is turned off. The count stored in the counter multiplied by the duration of each cycle of the reference clock is a measure of the delay.
More elaborate pulse type systems exist where the frequency is varied over the duration of the pulse. Such systems utilize a chirp pulse wherein the frequency of a wide bandwidth pulse is swept up or down over the duration of the pulse. The system's receiver is matched to the frequency sweep of the transmitted pulse so that the counter is turned off based upon the correlation of the transmitted and received pulse at frequency transition points. Another type of pulse system uses a (pseudo) random sequencing, such as a random binary code, to modulate the phase of the pulse. The phase coded pulse returned from the target is correlated against a replica of the code. The counter is stopped when a replica of the transmitted binary code sequence correlates to the binary code sequence of the received signal. Use of the conventional pulse type systems with target devices having signal bandwidth limitations is generally unacceptable because the degradation of the retransmitted limited bandwidth pulse results in poor ranging measurement accuracy.
Present continuous wave (CW) or range tone systems transmit a continuous signal to the target device over an extended period of time. The signal retransmitted from the target device to the ranging station is phase compared to the transmitted CW signal. The phase difference between the transmitted and received signals is utilized in determining the target range.
The use of a single frequency CW signal is not suitable for ranging over long distances since a multiple wavelength ambiguity is introduced when using the phase to determine the target range. Various techniques have been developed to improve the suitability of continuous wave systems for ranging purposes. One such technique is to transmit multiple tones of different frequency to resolve this ambiguity.
Careful choice of sequentially higher frequencies will enable the range accuracy to be improved while maintaining ambiguity resolution. However, CW systems must have prior knowledge of the estimated target range within the ambiguity resolution of the lowest frequency tone in order to be utilized. Furthermore, CW systems which use multiple tones require added time for range determination and therefore may provide poor ranging accuracy for fast moving target devices.
Both conventional pulse type and CW systems require that all ranging station components along the transmit and receive paths of the ranging signals be pre-calibrated for signal delay, and therefore any degradation or change in the components between the time of calibration and the time of actual range measurement will affect the accuracy of the measurement.
Thus, it is an object of the present invention to provide a system and method for accurately determining the range of a signal retransmitting target device with significant signal bandwidth restrictions. It is a further object of the present invention to provide a ranging system and method which does not require prior knowledge of the approximate target range. It is additionally an object of the present invention to provide a system and method for accurately determining the range of a fast moving target device. A still further object of the present invention is to provide a system and method which minimizes range determination error caused by pre-calibration of ranging station components.
While the present invention is described below with reference to a preferred embodiment for ranging to a geostationary communications satellite, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments within the scope of the present invention as disclosed and claimed herein and additional fields in which the present invention would be of significant utility.